Method and device of reselecting cell by terminal

ABSTRACT

Provided are a method of reselecting a cell by a terminal in a wireless communication system and a device supporting the same. A terminal may measure a cell on the basis of a priority, may calculate a ranking of the cell, using a cell measurement result, and may reselect a cell having the highest ranking. Otherwise, the terminal may measure a cell and evaluate a cell reselection criterion on the basis of a priority, may calculate a CE level with respect to a cell satisfying the cell reselection criterion evaluation, using a cell measurement result, and may reselect a cell having the lowest CE level. The terminal may be a terminal operating in a coverage extension mode and the coverage extension mode enables the cell measurement to be performed for a longer time than a normal mode.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system, andmore particularly, to a method of reselecting a cell by a terminal in awireless communication system, and a device supporting the method.

Related Art

3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) thatis an advancement of UMTS (Universal Mobile Telecommunication System) isbeing introduced with 3GPP release 8. In 3GPP LTE, OFDMA (orthogonalfrequency division multiple access) is used for downlink, and SC-FDMA(single carrier-frequency division multiple access) is used for uplink.The 3GPP LTE adopts MIMO (multiple input multiple output) having maximumfour antennas. Recently, a discussion of 3GPP LTE-A (LTE-Advanced) whichis the evolution of the 3GPP LTE is in progress.

A wireless communication system may need to estimate an uplink channelor downlink channel to transmit/receive data, to achieve systemsynchronization, and to feed back channel information. In a wirelesscommunication system environment, fading occurs by multipath time delay.A process of recovering a transmitted signal by compensating for asignal distortion caused by drastic environmental changes by fading isreferred to as channel estimation. Further, it is needed to measure achannel state with respect to a cell to which a user equipment (UE)belongs or another cell. For channel estimation or channel statemeasurement, channel estimation is generally performed using a referencesignal (RS) known between a transmitter and a receiver.

A UE may perform measurement using the following three methods.

1) Reference signal received power (RSRP): RSRP indicates the averagereceived power of all resource elements (REs) carrying CRSs transmittedover the entire band. Here, the UE may measure the average receivedpower of all REs carrying channel state information (CSI) RSs instead ofCRSs.

2) Received signal strength indicator (RSSI): RSSI indicates receivedpower measured over the entire band. RSSI includes all of a signal,interference, and thermal noise.

3) Reference symbol received quality (RSRQ): RSRQ indicates a channelquality indicator (CQI) and may be determined as RSRP/RSSI depending ona bandwidth or a sub-band. That is, RSRQ refers tosignal-to-interference-plus-noise-ratio (SINR). Since RSRP does notprovide sufficient mobility information, RSRQ may be used instead ofRSRP in a handover or cell reselection process.

RSRQ may be calculated by RSSI/RSSP. Alternatively, RSRQ may becalculated by N*RSSI/RSSP. Here, N may be a parameter (for example, thenumber of PRBs) or a function associated with a bandwidth in which RSSIis measured.

In recent years, machine-to-machine/Internet of Things (M2M/IoT), whichconnects all every objects through networks to facilitate obtaining andtransmitting necessary information anytime and anywhere, thereby makingit possible to offer and use various services, has become a major issuefor a next-generation communication market.

While early M2M started with a sensor and an RFID network mainly forlocal areas, various wired/wireless networks may be used with graduallydiversifying purposes and characteristics of applications. Recently, M2Mbased on a mobile communication network receives growing attention inview of the mobility of objects, a wide range of service areas includingnot only islands and mountains but also the sea, ease of networkmanagement and maintenance, security for reliable data transmission, andguarantee of service quality. Accordingly, with studies on thefeasibility of M2M started in 2005, the 3GPP has been conducting afull-scale standardization project under the name “Machine TypeCommunications (MTC)” since 2008.

The 3GPP regards a machine as an entity that does not require directhuman manipulation or intervention and defines MTC as a form of datacommunication involving one or more of machines. Typical examples of themachine include a smart meter and a vending machine that are equippedwith a mobile communication module. Recently, with the introduction of asmart phone that performs communication by automatically connecting to anetwork, without any user operation or intervention, depending on auser's location or conditions, a mobile terminal having an MTC functionis considered as a form of a machine. Also, a gateway-type MTC deviceconnected to an IEEE 802.15 WPAN-based subminiature sensor or RFID isalso considered.

SUMMARY OF THE INVENTION

Since the number of repetitions required for successful uplinktransmission and downlink reception is not considered in a cellreselection procedure, a terminal may reselect a cell requiring a greatnumber of repetitions in the cell reselection procedure, which may causea problem of excessive battery consumption or the like. Therefore, thepresent invention proposes a method of performing cell reselection byconsidering not only a priority but also a coverage enhancement level ora ranking, or performing cell reselection by considering the coverageenhancement level or the ranking without consideration of the priority.

According to an embodiment, there is provided a method of reselecting acell by a terminal in a wireless communication system. The method mayinclude: measuring a cell on the basis of a priority; calculating aranking of the cell by using a cell measurement result; and reselectinga cell having a highest ranking. The terminal may be a terminaloperating in a coverage enhancement (CE) mode.

The cell having the highest ranking may be a cell having a highestranking among neighboring cells having a higher ranking than a servingcell. The cell reselection may be performed during a timer durationTreselection and when one second elapses after the terminal moves to theserving cell.

In the terminal operating in the CE mode, a value calculated as a firstthreshold may not satisfy a cell selection condition, and a valuecalculated as a second threshold may satisfy the cell selectioncondition. The cell selection condition may be satisfied when valuesSrxlev and Squal exceed 0. The first threshold may be Qrxlevmin andQqualmin for calculating the values Srxlev and Squal in normal coverage.The second threshold may be Qrxlevmin_CE and Qqualmin_CE for calculatingthe values Srxlev and Squal in enhanced coverage. The first thresholdmay be greater than the second threshold.

The cell measurement may be performed in the CE mode for a longer timethan in the normal mode.

According to another embodiment, there is provided a method ofreselecting a cell by a terminal in a wireless communication system. Themethod may include: performing cell measurement and cell reselectioncriterion evaluation on the basis of the priority; calculating a CElevel by using a cell measurement result with respect to the cellsatisfying the cell reselection criterion evaluation; and performingcell reselection on a cell having a lowest CE level.

The cell having the lowest CE may be a cell having a highest ranking.

If there is a plurality of cells having a lowest CE level, the cellhaving the lowest CE level may be a cell having a top frequency priorityamong the cells having the lowest CE level.

The CE level may be calculated on the basis of measured reference signalreceived quality (RSRQ) or reference signal received power (RSRP). Acell having the lowest CE level may be a cell having the best measuredRSRQ or RSRP among cells satisfying the cell reselection criterionevaluation.

In case of a neighboring cell having a higher priority than the servingcell, if one second elapses after the terminal moves to the serving celland if a value obtained by subtracting reception quality of the servingcell from reception quality of the neighboring cell is greater than athreshold Thresh,HighQ during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. The timer Treselection and the threshold Thresh,HighQ may bereceived from the serving cell. The threshold Thresh,HighQ may be usedto determine whether the neighboring cell is a cell satisfying the cellreselection criterion evaluation on the basis of reception quality ofthe serving cell.

In case of a neighboring cell having a higher priority than the servingcell, if one second elapses after the terminal moves to the serving celland if a value obtained by subtracting reception quality of the servingcell from reception quality of the neighboring cell is greater than athreshold Thresh,HighP during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. The timer Treselection and the threshold Thresh,HighP may bereceived from the serving cell. The threshold Thresh,HighP may be usedto determine whether the neighboring cell is a cell satisfying the cellreselection criterion evaluation on the basis of reception quality ofthe serving cell.

In case of the neighboring cell having the higher priority than theserving cell, if one second elapses after the terminal moves to theserving cell and if a value obtained by subtracting the reception powerof the serving cell from the reception power of the neighboring cell isgreater than a threshold Thresh,HighLevel during the timer durationTreselection, then the neighboring cell may be a cell satisfying thecell reselection criterion evaluation. The timer Treselection and thethreshold Thresh,HighLevel may be received from the serving cell. Thethreshold Thresh,HighLevel may be used to determine whether theneighboring cell is a cell satisfying the cell reselection criterionevaluation on the basis of the CE level of the serving cell.

The terminal may be a terminal operating in the CE mode. The cellmeasurement may be performed in the CE mode for a longer time than inthe normal mode.

According to another embodiment, there is provided a terminal forreselecting a cell in a wireless communication system. The terminal mayinclude: a memory; a transceiver, and a processor operatively coupled tothe memory and the transceiver. The processor may be configured for:measuring a cell on the basis of a priority; calculating a ranking ofthe cell by using a cell measurement result; and reselecting a cellhaving a highest ranking. The terminal may be a terminal operating in aCE mode.

Since a terminal performs cell reselection by considering a coverageenhancement level or a ranking, it is possible to reselect a cellrequiring a small number of repetitions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wireless communication system to which the presentinvention is applied.

FIG. 2 shows a radio protocol architecture with respect to a user plane.

FIG. 3 shows a radio protocol architecture with respect to a controlplane.

FIG. 4 shows a procedure in which UE that is initially powered onexperiences a cell selection process, registers it with a network, andthen performs cell reselection if necessary.

FIG. 5 shows an RRC connection establishment procedure.

FIG. 6 shows an RRC connection reconfiguration procedure.

FIG. 7 shows an RRC connection re-establishment procedure.

FIG. 8 shows a conventional method of performing measurement.

FIG. 9 shows an example of a measurement configuration assigned to a UE.

FIG. 10 shows an example of deleting a measurement identity.

FIG. 11 shows an example of deleting the measurement object.

FIG. 12A and FIG. 12B show a method of (re)selecting a cell with an LTEneighboring frequency/neighboring UTRAN FDD frequency in a UE of anRRC_IDLE mode.

FIG. 13 shows an example of MTC.

FIG. 14 shows an example of cell coverage enhancement for an MTC device.

FIG. 15 shows an example of a cell reselection method considering a CElevel.

FIG. 16 shows another example of a cell reselection method considering aCE level.

FIG. 17 is a block diagram of a method of reselecting a cell by a UEaccording to an embodiment of the present invention.

FIG. 18 is a block diagram of a method of reselecting a cell by a UEaccording to an embodiment of the present invention.

FIG. 19 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses theOFDMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced(LTE-A) is an evolution of the LTE.

For clarity, the following description will focus on LTE-A. However,technical features of the present invention are not limited thereto.

FIG. 1 shows a wireless communication system to which the presentinvention is applied. The wireless communication system may also bereferred to as an evolved-UMTS terrestrial radio access network(E-UTRAN) or a long term evolution (LTE)/LTE-A system.

An E-UTRAN includes a base station (BS) 20 that provides a control planeand a user plane to a user equipment (UE) 10. The UE 10 may be fixed ormobile, and may be referred to by other name, such as a mobile station(MS), a user terminal (UT), User, a user equipment (UE), a subscriberstation (SS), a wireless device, or the like. The BS 20 generally refersto a fixed station that communicates with the UE 10 and may be called byother name, such as an eNB (evolved-Node B), a BTS (Base TransceiverSystem), an access point, or the like.

The BSs 20 may be connected with each other via an X2 interface. The BS20 is connected to an EPC (Evolved Packet Core) 30 via an S1 interface.Specifically, the BSs 20 are connected to an MME (Mobility ManagementEntity) via S1-MME and to an S-GW (Serving Gateway) via S1-U.

The EPC 30 includes an MME, an S-GW, and a P-GW (Packet DataNetwork-Gateway). The MME retains information regarding a UE access orinformation regarding UE capability, and such information is largelyused to manage UE mobility. The S-GW is a gateway having an E-UTRAN as aterminal point and the P-GW is a gateway having a PDN as a terminalpoint.

Layers of the radio interface protocols between a UE and a network maybe divided into a first layer L1, a second layer L2, and a third layerL3 based on the three lower layers of an open system interconnection(OSI) standard model widely known in communication systems. The physicallayer belonging to the first layer (L1) provides an information transferservice using a physical channel, and an RRC (Radio Resource Control)layer positioned in the third layer serves to control radio resourcesbetween the UE and the network. To this end, the RRC layer exchanges anRRC message between the UE and the network.

FIG. 2 shows a radio protocol architecture with respect to a user plane.FIG. 3 show a radio protocol architecture with respect to a controlplane. The user plane is a protocol stack for transmitting user data,and the control plane is a protocol stack for transmitting a controlsignal.

Referring to FIGS. 2 and 3, the physical layer provides an informationtransfer service to an upper layer by using a physical channel. Thephysical layer is connected to a medium access control (MAC) layer via atransport channel. Data is transferred between the MAC layer and thephysical layer via the transport channel Transport channels areclassified depending on how and with what kind of characteristics datais transmitted through a radio interface.

Between different physical layers, namely, between physical layers of atransmitter and a receiver, data is transferred via the physicalchannel. The physical channel may be modulated according to an OFDM(Orthogonal Frequency Division Multiplexing) scheme, and time andfrequency may be utilized as radio resources.

Functions of the MAC layer include mapping between a logical channel anda transport channel and multiplexing/demultiplexing an MAC SDU (servicedata unit) belonging to a logical channel into a transport blockbelonging to a physical channel via a transport channel. The MAC layerprovides a service to an RLC (radio link control) layer through alogical channel.

Functions of the RLC layer include concatenation, segmentation, andreassembly of an RLC SDU. In order to guarantee various QoS (Quality ofService) required by a radio bearer (RB), the RLC layer provides threetypes of operation modes: a transparent mode (TM), an unacknowledgedmode (UM), and an acknowledged mode (AM). An AM RLC provides an errorcorrection through an ARQ (automatic repeat request).

The RRC (Radio Resource Control) layer is defined only in the controlplane. The RRC layer handles controlling of a logical channel, atransport channel, and a physical channel in relation to configuration,re-configuration, and releasing of radio bearers (RBs). RB refers to alogical path provided by the first layer (PHY layer) and the secondlayers (MAC layer, RLC layer, and PDCP layer) to transfer data between aUE and a network.

A PDCP (Packet Data Convergence Protocol) layer in the user planeperforms a transfer of a user data, header compression, and ciphering.Functions of the PDCP in the control plane include transfer of controlplane data and ciphering/integrity protection.

Setting of an RB refers to defining characteristics of radio protocollayers and channels and configuring detailed parameters and operationmethods to provide a particular service. RBs may be divided into twotypes: SRB (Signaling RB) and DRB (Data RB). The SRB is used as apassage for transmitting an RRC message on the control plane, and theDRB is used as a passage for transferring an RRC message on the userplane.

When there is an RRC connection between the RRC of the UE and that ofthe E-UTRAN, the UE is in an RRC-connected mode, or otherwise, the UE isin an RRC idle mode.

Downlink transport channels for transmitting data from the network tothe UE includes a BCH (Broadcast Channel) for transmitting systeminformation and a DL-SCH (Downlink-Shared Channel) for transmitting usertraffic or a control message. Traffic or a control message of a downlinkmulticast or broadcast service may be transmitted via the DL-SCH or anextra downlink MCH (Multicast Channel). Meanwhile, uplink transportchannels for transmitting data from the UE to the network includes anRACH (Random Access Channel) for transmitting an initial control messageand a UL-SCH (Uplink-Shared Channel) for transmitting user traffic or acontrol message.

Logical channels positioned at a higher level and mapped to a transportchannel includes a BCCH (Broadcast Channel), a PCCH (Paging ControlChannel), a CCCH (Common Control Channel), an MCCH (Multicast ControlChannel), an MTCH (Multicast Traffic Channel), or the like.

A physical channel is comprised of several OFDM symbols in a time domainand several subcarriers in a frequency domain. A single subframeincludes a plurality of OFDM symbols in the time domain. A resource bockis a resource allocation unit, which includes a plurality of OFDMsymbols and a plurality of subcarriers. Each subframe may use particularsubcarriers of particular OFDM symbols (e.g., first OFDM symbol) of acorresponding subframe for a PDCCH (Physical Downlink Control Channel),namely, for an L1/L2 control channel. A TTI (Transmission Time Interval)is a unit time of a subframe transmission.

Hereinafter, an RRC state and an RRC connection method will bedescribed.

An RRC state refers to whether or not an RRC layer of a UE is logicallyconnected to that of the E-UTRAN. When the RRC layer of the UE islogically connected to that of the E-UTRAN, it is called an RRCconnected state, and or otherwise, it is called an RRC idle state. Whenthe UE is in the RRC connected state, since the RRC connection exists,the E-UTRAN can recognize the presence of the corresponding UE by cell,and thus, the E-UTRAN can effectively control the UE. Meanwhile, whenthe UE is in the RRC idle state, the E-UTRAN cannot recognize the UE inthe RRC idle state, and the UE is managed by a core network (CN) bytrack area unit larger than a cell. Namely, the UE in the RRC idle stateis recognized as to whether or not it is present by the larger areaunit, and in order for the UE in the RRC idle state to receive a generalmobile communication service such as a voice or data, the UE in the RRCidle state is to be changed to the RRC connected state.

When the user first turns on power of a UE, the UE first searches for anappropriate cell and remains in an RRC idle state in the correspondingcell. When the UE in the RRC idle state is required to be RRC connected,the UE establishes an RRC connection with the E-UTRAN through an RRCconnection procedure and transitions to the RRC connection state. Whenthe UE in the RRC idle state may need to establish an RRC connection forvarious reasons. For example, the UE in the RRC idle state may establishan RRC connection when a transmission of uplink data is required as theuser attempts to make a call, or the like, or when a response isrequired to be transmitted as a paging message is received from theE-UTRAN.

A NAS (Non-Access Stratum) layer positioned at a higher level of the RRClayer performs functions such as session management, mobilitymanagement, and the like.

In order for the NAS layer to manage mobility of the UE, two states ofEMM-REGISTERED (EPS Mobility Management-REGISTERED) and EMM-DEREGISTEREDare defined, and these two states are applied to a UE and an MME.Initially, the UE is in the EMM-DEREGISTERED state, and in order toaccess a network, the UE performs a process of registering thecorresponding network through an initial attach procedure. When theattach procedure is successfully performed, the UE and the MME are inthe EMM-REGISTERED state.

In order to manage a signaling connection between the UE and the EPC,two states of ECM (EPS Connection Management)-IDLE and an ECM-CONNECTEDstate are defined, and these two states are applied to the UE and theMME. When the UE in the ECM-IDLE state establishes an RRC connectionwith the E-UTRAN, the corresponding UE is in the ECM-CONNECTED state.When the MME in the ECM-IDLE state establishes an S1 connection with theE-UTRAN, the MME is in an ECM-CONNECTED state. When the UE is in theECM-IDLE state, the E-UTRAN does not have information regarding contextof the UE. Thus, the UE in the ECM-IDLE state performs a UE-basedmobility related procedure such as cell selection or cell reselectionwithout receiving a command of the network. Meanwhile, when the UE is inthe ECM-CONNECTED state, mobility of the UE is managed by a command ofthe network. In the ECM-IDLE state, when a location of the UE is changedto be different from that known by the network, the UE informs thenetwork about its location through a tracking area updating procedure.

FIG. 4 shows a procedure in which UE that is initially powered onexperiences a cell selection process, registers it with a network, andthen performs cell reselection if necessary.

Referring to FIG. 4, the UE selects Radio Access Technology (RAT) inwhich the UE communicates with a Public Land Mobile Network (PLMN), thatis, a network from which the UE is provided with service (S410).Information about the PLMN and the RAT may be selected by the user ofthe UE, and the information stored in a Universal Subscriber IdentityModule (USIM) may be used.

The UE selects a cell that has the greatest value and that belongs tocells having measured BS and signal intensity or quality greater than aspecific value (cell selection) (S420). In this case, the UE that ispowered off performs cell selection, which may be called initial cellselection. A cell selection procedure is described later in detail.After the cell selection, the UE receives system informationperiodically by the BS. The specific value refers to a value that isdefined in a system in order for the quality of a physical signal indata transmission/reception to be guaranteed. Accordingly, the specificvalue may differ depending on applied RAT.

If network registration is necessary, the UE performs a networkregistration procedure (S430). The UE registers its information (e.g.,an IMSI) with the network in order to receive service (e.g., paging)from the network. The UE does not register it with a network whenever itselects a cell, but registers it with a network when information aboutthe network (e.g., a Tracking Area Identity (TAI)) included in systeminformation is different from information about the network that isknown to the UE.

The UE performs cell reselection based on a service environment providedby the cell or the environment of the UE (S440). If the value of theintensity or quality of a signal measured based on a BS from which theUE is provided with service is lower than that measured based on a BS ofa neighboring cell, the UE selects a cell that belongs to other cellsand that provides better signal characteristics than the cell of the BSthat is accessed by the UE. This process is called cell reselectiondifferently from the initial cell selection of the No. 2 process. Inthis case, temporal restriction conditions are placed in order for acell to be frequently reselected in response to a change of signalcharacteristic. A cell reselection procedure is described later indetail.

FIG. 5 shows an RRC connection establishment procedure.

The UE sends an RRC connection request message that requests RRCconnection to a network (S510). The network sends an RRC connectionestablishment message as a response to the RRC connection request(S520). After receiving the RRC connection establishment message, the UEenters RRC connected mode.

The UE sends an RRC connection establishment complete message used tocheck the successful completion of the RRC connection to the network(S530).

FIG. 6 shows an RRC connection reconfiguration procedure.

An RRC connection reconfiguration is used to modify RRC connection. Thisis used to establish/modify/release RBs, perform handover, and setup/modify/release measurements.

A network sends an RRC connection reconfiguration message for modifyingRRC connection to UE (S610). As a response to the RRC connectionreconfiguration message, the UE sends an RRC connection reconfigurationcomplete message used to check the successful completion of the RRCconnection reconfiguration to the network (S620).

The following is a detailed description of a procedure of selecting acell by a terminal.

When power is turned-on or the terminal is located in a cell, theterminal performs procedures for receiving a service byselecting/reselecting a suitable quality cell.

A terminal in an RRC idle state should prepare to receive a servicethrough the cell by always selecting a suitable quality cell. Forexample, a terminal where power is turned-on just before should selectthe suitable quality cell to be registered in a network. If the terminalin an RRC connection state enters in an RRC idle state, the terminalshould selects a cell for stay in the RRC idle state. In this way, aprocedure of selecting a cell satisfying a certain condition by theterminal in order to be in a service idle state such as the RRC idlestate refers to cell selection. Since the cell selection is performed ina state that a cell in the RRC idle state is not currently determined,it is important to select the cell as rapid as possible. Accordingly, ifthe cell provides a wireless signal quality of a predetermined level orgreater, although the cell does not provide the best wireless signalquality, the cell may be selected during a cell selection procedure ofthe terminal.

Hereinafter, a method and a procedure of selecting a cell by a terminalin a 3GPP LTE is described.

A cell selection process is basically divided into two types.

The first is an initial cell selection process. In this process, UE doesnot have preliminary information about a wireless channel. Accordingly,the UE searches for all wireless channels in order to find out a propercell. The UE searches for the strongest cell in each channel Thereafter,if the UE has only to search for a suitable cell that satisfies a cellselection criterion, the UE selects the corresponding cell.

Next, the UE may select the cell using stored information or usinginformation broadcasted by the cell. Accordingly, cell selection may befast compared to an initial cell selection process. If the UE has onlyto search for a cell that satisfies the cell selection criterion, the UEselects the corresponding cell. If a suitable cell that satisfies thecell selection criterion is not retrieved though such a process, the UEperforms an initial cell selection process.

After the UE selects a specific cell through the cell selection process,the intensity or quality of a signal between the UE and a BS may bechanged due to a change in the mobility or wireless environment of theUE. Accordingly, if the quality of the selected cell is deteriorated,the UE may select another cell that provides better quality. If a cellis reselected as described above, the UE selects a cell that providesbetter signal quality than the currently selected cell. Such a processis called cell reselection. In general, a basic object of the cellreselection process is to select a cell that provides UE with the bestquality from a viewpoint of the quality of a radio signal.

In addition to the viewpoint of the quality of a radio signal, a networkmay determine priority corresponding to each frequency, and may informthe UE of the determined priorities. The UE that has received thepriorities preferentially takes into consideration the priorities in acell reselection process compared to a radio signal quality criterion.

As described above, there is a method of selecting or reselecting a cellaccording to the signal characteristics of a wireless environment. Inselecting a cell for reselection when a cell is reselected, thefollowing cell reselection methods may be present according to the RATand frequency characteristics of the cell.

-   -   Intra-frequency cell reselection: UE reselects a cell having the        same center frequency as that of RAT, such as a cell on which        the UE camps on.    -   Inter-frequency cell reselection: UE reselects a cell having a        different center frequency from that of RAT, such as a cell on        which the UE camps on    -   Inter-RAT cell reselection: UE reselects a cell that uses RAT        different from RAT on which the UE camps

The principle of a cell reselection process is as follows.

First, UE measures the quality of a serving cell and neighbor cells forcell reselection.

Second, cell reselection is performed based on a cell reselectioncriterion. The cell reselection criterion has the followingcharacteristics in relation to the measurements of a serving cell andneighbor cells.

Intra-frequency cell reselection is basically based on ranking. Rankingis a task for defining a criterion value for evaluating cell reselectionand numbering cells using criterion values according to the size of thecriterion values. A cell having the best criterion is commonly calledthe best-ranked cell. The cell criterion value is based on the value ofa corresponding cell measured by UE, and may be a value to which afrequency offset or cell offset has been applied, if necessary.

Inter-frequency cell reselection is based on frequency priority providedby a network. UE attempts to camp on a frequency having the highestfrequency priority. A network may provide frequency priority that willbe applied by UEs within a cell in common through broadcastingsignaling, or may provide frequency-specific priority to each UE throughUE-dedicated signaling. A cell reselection priority provided throughbroadcast signaling may refer to a common priority. A cell reselectionpriority for each terminal set by a network may refer to a dedicatedpriority. If receiving the dedicated priority, the terminal may receivea valid time associated with the dedicated priority together. Ifreceiving the dedicated priority, the terminal starts a validity timerset as the received valid time together therewith. While the valid timeris operated, the terminal applies the dedicated priority in the RRC idlemode. If the valid timer is expired, the terminal discards the dedicatedpriority and again applies the common priority.

For the inter-frequency cell reselection, a network may provide UE witha parameter (e.g., a frequency-specific offset) used in cell reselectionfor each frequency.

For the intra-frequency cell reselection or the inter-frequency cellreselection, a network may provide UE with a Neighboring Cell List (NCL)used in cell reselection. The NCL includes a cell-specific parameter(e.g., a cell-specific offset) used in cell reselection.

For the intra-frequency or inter-frequency cell reselection, a networkmay provide UE with a cell reselection black list used in cellreselection. The UE does not perform cell reselection on a cell includedin the black list.

Ranking performed in a cell reselection evaluation process is describedbelow.

A ranking criterion used to apply priority to a cell is defined as inEquation 1.

R _(S)=

_(meas,s)+

_(hyst) , R _(n)=

_(meas,n)−

_(offset)  [Equation 1]

In this case, Rs is the ranking criterion of a serving cell, Rn is theranking criterion of a neighbor cell, Qmeas,s is the quality value ofthe serving cell measured by UE, Qmeas,n is the quality value of theneighbor cell measured by UE, Qhyst is the hysteresis value for ranking,and Qoffset is an offset between the two cells.

In Intra-frequency, if UE receives an offset “Qoffsets,n” between aserving cell and a neighbor cell, Qoffset=Qoffsets,n. If UE does notQoffsets,n, Qoffset=0.

In Inter-frequency, if UE receives an offset “Qoffsets,n” for acorresponding cell, Qoffset=Qoffsets,n+Qfrequency. If UE does notreceive “Qoffsets,n”, Qoffset=Qfrequency.

If the ranking criterion Rs of a serving cell and the ranking criterionRn of a neighbor cell are changed in a similar state, ranking priorityis frequency changed as a result of the change, and UE may alternatelyreselect the twos. Qhyst is a parameter that gives hysteresis to cellreselection so that UE is prevented from to alternately reselecting twocells.

UE measures RS of a serving cell and Rn of a neighbor cell according tothe above equation, considers a cell having the greatest rankingcriterion value to be the best-ranked cell, and reselects the cell. If areselected cell is not a suitable cell, UE excludes a correspondingfrequency or a corresponding cell from the subject of cell reselection.

FIG. 7 shows an RRC connection re-establishment procedure.

Referring to FIG. 7, UE stops using all the radio bearers that have beenconfigured other than a Signaling Radio Bearer (SRB) #0, and initializesa variety of kinds of sublayers of an Access Stratum (AS) (S710).Furthermore, the UE configures each sublayer and the PHY layer as adefault configuration. In this procedure, the UE maintains the RRCconnection state.

The UE performs a cell selection procedure for performing an RRCconnection reconfiguration procedure (S720). The cell selectionprocedure of the RRC connection re-establishment procedure may beperformed in the same manner as the cell selection procedure that isperformed by the UE in the RRC idle state, although the UE maintains theRRC connection state.

After performing the cell selection procedure, the UE determines whetheror not a corresponding cell is a suitable cell by checking the systeminformation of the corresponding cell (S730). If the selected cell isdetermined to be a suitable E-UTRAN cell, the UE sends an RRC connectionre-establishment request message to the corresponding cell (S740).

Meanwhile, if the selected cell is determined to be a cell that uses RATdifferent from that of the E-UTRAN through the cell selection procedurefor performing the RRC connection re-establishment procedure, the UEstops the RRC connection re-establishment procedure and enters the RRCidle state (S750).

The UE may be implemented to finish checking whether the selected cellis a suitable cell through the cell selection procedure and thereception of the system information of the selected cell. To this end,the UE may drive a timer when the RRC connection re-establishmentprocedure is started. The timer may be stopped if it is determined thatthe UE has selected a suitable cell. If the timer expires, the UE mayconsider that the RRC connection re-establishment procedure has failed,and may enter the RRC idle state. Such a timer is hereinafter called anRLF timer. In LTE spec TS 36.331, a timer named “T311” may be used as anRLF timer. The UE may obtain the set value of the timer from the systeminformation of the serving cell.

If an RRC connection re-establishment request message is received fromthe UE and the request is accepted, a cell sends an RRC connectionre-establishment message to the UE.

The UE that has received the RRC connection re-establishment messagefrom the cell reconfigures a PDCP sublayer and an RLC sublayer with anSRB1. Furthermore, the UE calculates various key values related tosecurity setting, and reconfigures a PDCP sublayer responsible forsecurity as the newly calculated security key values. Accordingly, theSRB 1 between the UE and the cell is open, and the UE and the cell mayexchange RRC control messages. The UE completes the restart of the SRB1,and sends an RRC connection re-establishment complete message indicativeof that the RRC connection re-establishment procedure has been completedto the cell (S760).

In contrast, if the RRC connection re-establishment request message isreceived from the UE and the request is not accepted, the cell sends anRRC connection re-establishment reject message to the UE.

If the RRC connection re-establishment procedure is successfullyperformed, the cell and the UE perform an RRC connection reconfigurationprocedure. Accordingly, the UE recovers the state prior to the executionof the RRC connection re-establishment procedure, and the continuity ofservice is guaranteed to the upmost.

FIG. 8 shows a conventional method of performing measurement.

A UE receives measurement configuration information from a BS (S810). Amessage including the measurement configuration information is referredto as a measurement configuration message. The UE performs measurementbased on the measurement configuration information (S820). If ameasurement result satisfies a reporting condition included in themeasurement configuration information, the UE reports the measurementresult to the BS (S830). A message including the measurement result isreferred to as a measurement report message.

The measurement configuration information may include the followinginformation.

(1) Measurement object: The object is on which the UE performs themeasurements. The measurement object includes at least one of anintra-frequency measurement object which is an object of intra-frequencymeasurement, an inter-frequency measurement object which is an object ofinter-frequency measurement, and an inter-RAT measurement object whichis an object of inter-RAT measurement. For example, the intra-frequencymeasurement object may indicate a neighboring cell having the samefrequency as a frequency of a serving cell, the inter-frequencymeasurement object may indicate a neighboring cell having a differentfrequency from a frequency of the serving cell, and the inter-RATmeasurement object may indicate a neighboring cell of a different RATfrom an RAT of the serving cell.

(2) Reporting configuration: This includes a reporting criterion and areporting format. The reporting criterion is used to trigger the UE tosend a measurement report and can either be periodical or a single eventdescription. The reporting format is a quantity that the UE includes inmeasurement reporting and associated information (e.g. number of cellsto report).

(3) Measurement identify: Each measurement identity links onemeasurement object with one reporting configuration. By configuringmultiple measurement identities, it is possible to link more than onemeasurement object to the same reporting configuration, as well as tolink more than one reporting configuration to the same measurementobject. The measurement identity is used as a reference number inmeasurement reporting. The measurement identify may be included inmeasurement reporting to indicate a specific measurement object forwhich the measurement result is obtained and a specific reportingcondition according to which measurement reporting is triggered.

(4) Quantity configuration: One quantity configuration is configured perRAT type. The quantity configuration defines the measurement quantitiesand associated filtering used for all event evaluation and relatedreporting of that measurement type. One filter can be configured permeasurement quantity.

(5) Measurement gaps: Measurement gaps are periods that the UE may useto perform measurements when downlink transmission and uplinktransmission are not scheduled.

To perform a measurement procedure, the UE has a measurement object, areporting configuration, and a measurement identity.

In 3GPP LTE, the BS can assign only one measurement object to the UEwith respect to one frequency. Events for triggering measurementreporting are shown in the table 1. If the measurement result of the UEsatisfies the determined event, the UE transmits a measurement reportmessage to the BS.

TABLE 1 Event Reporting Condition Event A1 Serving becomes better thanthreshold Event A2 Serving becomes worse than threshold Event A3Neighbour becomes offset better than serving Event A4 Neighbour becomesbetter than threshold Event A5 Serving becomes worse than threshold1 andneighbour becomes better than threshold2 Event B1 Inter RAT neighbourbecomes better than threshold Event B2 Serving becomes worse thanthreshold1 and inter RAT neighbour becomes better than threshold2

FIG. 9 shows an example of a measurement configuration assigned to a UE.

First, a measurement identity1 901 associates an intra-frequencymeasurement object with a reporting configuration 1. The UE performsintra-frequency measurement. The reporting configuration 1 is used todetermine a reporting type and a criterion for reporting a measurementresult.

A measurement identity2 902 is associated with the intra-frequencymeasurement object similarly to the measurement identifier1 901, andassociates the intra-frequency measurement object with a reportingconfiguration 2. The UE performs intra-frequency measurement. Thereporting configuration 2 is used to determine a reporting format and acriterion for reporting a measurement result.

By using the measurement identity1 901 and the measurement identity2902, the UE transmits a measurement result even if the measurementresult on the intra-frequency measurement object satisfies any one ofthe reporting configuration 1 and the reporting configuration 2.

A measurement identity3 903 associates an inter-frequency measurementobject 1 with a reporting configuration 3. When a measurement result onthe inter-frequency measurement object 1 satisfies a reporting criterionincluded in the reporting configuration 1, the UE reports themeasurement result.

A measurement identity4 904 associates an inter-frequency measurementobject 2 with the reporting configuration 2. When a measurement resulton the inter-frequency measurement object 2 satisfies a reportingcriterion included in the reporting configuration 2, the UE reports themeasurement result.

Meanwhile, the measurement object, the reporting configuration, and/orthe measurement identity can be added, modified, and/or deleted. Toinstruct such operations, the BS may transmit to the UE a newmeasurement configuration message or a measurement configurationmodification message.

FIG. 10 shows an example of deleting a measurement identity. When themeasurement identity 2 902 is deleted, measurement for a measurementobject associated with the measurement identity 2 902 is stopped and ameasurement report is not transported. A measurement object or areporting configuration associated with the deleted measurement identitymay not be changed.

FIG. 11 shows an example of deleting the measurement object. When theinter-frequency measurement object 1 is deleted, the UE deletes even themeasurement identity 3 903 associated therewith. Measurement for theinter-frequency measurement object 1 is stopped and a measurement reportis not also transported. However, a reporting configuration associatedwith the deleted inter-frequency measurement object may not be changedor deleted.

When the reporting configuration is removed, the UE removes even ameasurement identity associated therewith. The UE stops measurement foran associated measurement object by the associated measurement identity.However, the measurement object associated with the deleted reportingconfiguration may not be changed or deleted.

The measurement report may include the measurement identity, a measuredquality of the serving cell, and a measurement result of the neighborcell. The measurement identity identifies a measurement object in whichthe measurement report is triggered. The measurement result of theneighbor cell may include a cell identity and a measurement quality ofthe neighbor cell. The measured quality may include at least one ofreference signal received power (RSRP) and reference signal receivedquality (RSRQ).

Hereinafter, an inter-frequency or inter-RAT cell reselection criterionwill be described in greater detail.

FIG. 12A and FIG. 12B show a method of (re)selecting a cell with an LTEneighboring frequency/neighboring UTRAN FDD frequency in a UE of anRRC_IDLE mode.

In a state where an RRC connection is not established with a BS forcontrolling a cell, the UE of the RRC_IDLE mode periodically receivessome common channels such as paging to ensure mobility of the UE byselecting a proper cell according to a channel environment. The 3GPPspecification of ‘TS36.304 E-UTRA UE Procedures in idle mode’ is used byreference for description on a detailed operation of the UE of theRRC_IDLE mode in a 3GPP system.

Referring to FIG. 12A and FIG. 12B, when the UE of the RRC_IDLE modestarts to perform measurement for cell reselection with a neighboringLTE frequency or a neighboring UTRAN FDD frequency (S1201), it ischecked whether a signal quality related parameter ThreshServingLowQ isprovided through system information block (SIB) 3 as one of systeminformation which is broadcast within a cell (S1211). The signal qualityrelated parameter ThreshServingLowQ is a comparative threshold valuewhich is signaled through the SIB 3 to determine a channel state Squalof a current serving cell as one condition for moving to an LTEneighboring frequency or another system's frequency having a lowerpriority than a current serving frequency. Squal will be described at alater time. If the signal quality related parameter ThreshServingLowQ isnot signaled (i.e., is not provided) through the SIB 3, a measuredmeasurement result and system information which is broadcast in aserving LTE cell are applied to derive a value Srxlev of the servingcell and a value Srxlev of a corresponding neighboring LTEfrequency/UTRAN FDD frequency cell (S1221). Srxlev denotes a cellselection RX level value (dB), and is defined as shown in Equation 2below.

Srxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−Pcompensation−Qoffsettem  [Equation2]

Qrxlevmeas denotes a downlink reception power value used when the UEactually measures an RX channel, Qrxlevmin denotes a minimum downlinkreceiver power requirement level required to select a correspondingcell, Qrxlevminoffset denotes a threshold value to be added to Qrxlevminonly when the UE periodically searches for a public land mobile network(PLMN, or a communication vendor) having a higher priority while beingpresent in a visited public land mobile network (VPLMN), Pcompensationis a threshold value considering an uplink channel state, andQoffsettemp is an offset temporarily applied to the cell. Each parameteris described as shown in Table 2 below.

TABLE 2 Srxlev Cell selection RX level value (dB) Qrxlevmeas Measuredcell RX level value (RSRP) Qrxlevmin Minimum required RX level in thecell (dBm) Qrxlevminoffset Offset to the signalled Qrxlevmin taken intoaccount in the Srxlev evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in a VPLMN Pcompensationmax(PEMAX − PPowerClass, 0) (dB) PEMAX Maximum TX power level a UE mayuse when transmitting on the uplink in the cell (dBm) defined as PEMAXin [TS 36.101] PPowerClass Maximum RF output power of the UE (dBm)according to the UE power class as defined in [TS 36.101] QoffsettempOffset temporarily applied to a cell as specified in [TS 36.331]

Qrxlevmeas of a serving LTE cell and a cell of a neighboring LTEfrequency is obtained as reference signal received power (RSRP), andQrxlevmeas of a cell of a neighboring UTRAN frequency is obtained asreceived signal code power (RSCP). The specification ‘TS36.214 E-UTRAPhysical Layer Measurements’ of 3GPP is used by reference for a moredetailed definition on the RSRP, and the specification ‘TS25.215Physical Layer—Measurements (FDD)’ of 3GPP is used by reference for amore detailed definition on the RSCP. Qrxlevmin for the serving LTE cellapplies a value Q-RxLevMin received in a system information block (SIB)3 as one of system information which is broadcast in the serving LTE.Qrxlevmin for the cell of the neighboring LTE frequency applies a valueQ-RxLevMin to be mapped to a corresponding frequency received in an SIB5 as one of system information which is broadcast in the serving LTE.Qrxlevmin for a neighboring UTRAN FDD frequency cell applies a valueq-RxLevMin to be mapped to a corresponding frequency received in an SIB6 as one of system information which is broadcast in the serving LTEcell. That is, the value Qrxlevmin may be applied as a different valueto each frequency when obtaining Srxlev for the serving LTE cell, Srxlevfor the cell of the neighboring LTE frequency, and Srxlev for the cellof the neighboring UTRAN FDD frequency. PEMAX applied to obtainPcompensation denotes permitted maximum transmission power configuredfor a UE by a radio network controller (RNC). Similarly to Qrxlevmin,the serving LTE cell applies a value P-Max received in the SIB 3, andthe cell of the neighboring LTE frequency applies a value P-Max receivedin the SIB 5, and the cell of the neighboring UTRAN FDD frequencyapplies a value P-Max received in the SIB 6. Ppowerclass denotes actualmaximum RF transmission power of the UE. The value Ppowerclass may havea different value for each class of the UE.

The UE checks whether a neighboring LTE frequency/UTRAN FDD frequency tobe measured has a higher priority than the serving LTE frequency(S1231). The priority information indicates a specific frequency whichis considered preferentially by the UE to perform cell reselectionthereon, and may be received as system information which is broadcast inthe serving LTE cell or a dedicated message (e.g., an RRC connectionrelease) when the UE is in the RRC connected state.

If the neighboring LTE frequency/UTRAN FDD frequency to be measured hasa higher priority than the current serving LTE frequency (S1231), if thevalue Srxlev of the cell of the neighboring LTE frequency/UTRAN FDDfrequency is higher than a value ThreshX,HighP during a timer durationTreselection (S1233), and if one second elapses after moving to thecurrent serving LTE cell (S1235), then cell reselection is performed ona cell of a corresponding neighboring LTE frequency/UTRAN FDD frequency(S1237). The timer value Treselection to be applied to the neighboringLTE frequency/UTRAN FDD frequency and a comparative threshold valueThreshX,HighP Srxlev are received as system information which isbroadcast in the serving LTE cell.

Meanwhile, if the neighboring LTE frequency/UTRAN FDD frequency to bemeasured is not higher than (i.e., equal to or lower than) the currentserving LTE frequency (S1231), if the value Srxlev of the cell of theneighboring LTE frequency/UTRAN FDD frequency is higher than a valueThreshX,LowP and the value Srxlev of the serving cell of the current LTEfrequency is less than a value ThreshServing,LowP during the timeduration Treselection (S1241), and if one second elapses after moving tothe current serving LTE cell (S1235), then cell reselection is performedon a cell of a corresponding neighboring LTE frequency/UTRAN FDDfrequency (S1237). If the above condition is not satisfied, the cellreselection does not occur with the neighboring LTE frequency/UTRAN FDDfrequency.

If the value ThreshServingLowQ is signaled/provided through the SIB 3 ofthe serving LTE cell (S1211), the measured measurement result and thesystem information which is broadcast in the serving LTE cell areapplied to derive a value Squal of the serving cell and a value Squal ofa corresponding neighboring LTE frequency/UTRAN FDD frequency cell(S1251). Squal denotes a cell selection quality value (dB), and isdefined as shown in Equation 3 below.

Squal=Qqualmeas−(Qqualmin+Qqualminoffset)−Qoffsettemp  [Equation 3]

Qqualmeas denotes a value obtained by calculating a ratio of receivedsignal strength used when the UE actually measures a downlink RX channeland total noise actually measured. Qqualmin a minimum signal to noiseratio level required to select a corresponding cell. Qqualminoffsetdenotes a threshold value to be added to Qqualmin only when the UEperiodically searches for a PLMN having a higher priority while beingpresent in a VPLMN, and Qoffsettemp is an offset temporarily applied tothe cell. Each parameter is described as shown in Table 3 below.

TABLE 3 Squal Cell selection quality value (dB) Qqualmeas Measured cellquality value (RSRQ) Qqualmin Minimum required quality level in the cell(dB) Qqualminoffset Offset to the signalled Qqualmin taken into accountin the Squal evaluation as a result of a periodic search for a higherpriority PLMN while camped normally in a VPLMN Qoffsettemp Offsettemporarily applied to a cell as specified in [TS 36.331]

Qqualmeas of a serving LTE cell and a cell of a neighboring LTEfrequency is obtained as reference signal received quality (RSRQ), andQqualmeas of a cell of a neighboring UTRAN frequency is obtained asEc/No (obtained as RSCP/RSSI, RSSI: Received Signal Strength Indicator).The specification ‘TS36.214 E-UTRA Physical Layer Measurements’ of 3GPPis used by reference for a more detailed definition on the RSRQ, and thespecification ‘TS25.215 Physical Layer—Measurements (FDD)’ of 3GPP isused by reference for a more detailed definition on the Ec/No. Qqualminfor the serving LTE cell applies a value Q-QualMin-r9 received in an SIB3 as one of system information which is broadcast in the serving LTE.Qqualmin for the cell of the neighboring LTE frequency applies a valueQ-QualMin-r9 to be mapped to a corresponding frequency received in anSIB 5 as one of system information which is broadcast in the servingLTE. Qqualmin for a neighboring UTRAN FDD frequency cell applies a valueq-QualMin to be mapped to a corresponding frequency received in an SIB 6as one of system information which is broadcast in the serving LTE cell.That is, the value Qqualmin may be applied as a different value to eachfrequency when obtaining Squal for the serving LTE cell, Squal for thecell of the neighboring LTE frequency, and Squal for the cell of theneighboring UTRAN FDD frequency.

The UE checks whether a neighboring LTE frequency/UTRAN FDD frequency tobe measured has a higher priority than the serving LTE frequency(S1261). The priority information indicates a specific frequency whichis considered preferentially by the UE to perform cell reselectionthereon, and may be received as system information which is broadcast inthe serving LTE cell or a dedicated message (e.g., an RRC connectionrelease) when the UE is in the RRC connected state.

If the neighboring LTE frequency/UTRAN FDD frequency to be measured hasa higher priority than the current serving LTE frequency (S1261), if thevalue Srxlev of the cell of the neighboring LTE frequency/UTRAN FDDfrequency is higher than a value ThreshX,HighQ during a timer durationTreselection (S1263), and if one second elapses after moving to thecurrent serving LTE cell (S1265), then cell reselection is performed ona cell of a corresponding neighboring LTE frequency/UTRAN FDD frequency(S1267). The timer value Treselection to be applied to the neighboringLTE frequency/UTRAN FDD frequency and a comparative threshold valueThreshX,HighQ Srxlev are received as system information which isbroadcast in the serving LTE cell.

Meanwhile, if the neighboring LTE frequency/UTRAN FDD frequency to bemeasured is not higher than (i.e., equal to or lower than) the currentserving LTE frequency (S1261), if the value Srxlev of the cell of theneighboring LTE frequency/UTRAN FDD frequency is higher than a valueThreshX,LowQ and the value Srxlev of the serving cell of the current LTEfrequency is less than a value ThreshServing,LowQ during the timeduration Treselection (S1271), and if one second elapses after moving tothe current serving LTE cell (S1265), then cell reselection is performedon a cell of a corresponding neighboring LTE frequency/UTRAN FDDfrequency (S1267). If the above condition is not satisfied, the cellreselection does not occur with the neighboring LTE frequency/UTRAN FDDfrequency.

Further, the UE may search for a closed subscriber group (CSG) cellincluding the UE as its member during the normal cell reselectionprocedure. If a CSG cell suitable for another frequency is discovered,cell reselection is performed on the CSG cell regardless of a priorityof a frequency at which the UE currently resides. That is, the CSG cellhas a top priority in the cell reselection.

Meanwhile, among UEs, a UE for receiving a multimedia broadcast andmulticast service (MBMS) service may assign the top priority to acurrently served frequency. Further, among the UEs, UEs intending toreceive an MBMS service may assign the top priority to a frequency atwhich the MBMS service is broadcast. In addition, a UE which is nolonger able to receive a service at a corresponding frequency or whichhas completed the service may not regulate a priority for the frequencyat which the MBMS service is received.

Hereinafter, a cell selection criterion S will be described in greaterdetail.

The cell selection criterion used by a UE in cell selection is definedas shown in Equation 4 below.

Srxlev>0 and Squal>0  [Equation 4]

Srxlev is defined as shown in Equation 2 above, and Squal is defined asshown in Equation 3 above. Referring to Equation 4 above, the cellselection criterion may be satisfied when both of Srxlev and Squal aregreater than 0. That is, when both of the RSRP and RSRQ of the measuredcell are greater than or equal to a specific level, the UE may determinethe cell as a cell having a basic possibility for cell reselection. Inparticular, Squal is a parameter corresponding to the RSRQ. That is,Squal is a value calculated in association with quality of power ratherthan simply a value associated with a magnitude of power measured in thecell. The cell selection criterion may be satisfied in terms of qualityof the cell if Squal>0. The cell selection criterion for the RSRQ may besatisfied only when the measured RSRQ is greater than or equal to a sumof Qqualmin and Qqualminoffset.

Hereinafter, machine-type communication (MTC) will be described.

FIG. 13 shows an example of MTC.

MTC refers to information exchange between MTC UEs 1310 via a BS 1320without involving human interactions or information exchanges between anMTC UE 1310 and an MTC server 1330 via the BS. Services provided throughMTC are differentiated from existing communication services requiringhuman intervention, and MTC provides a wide range of services, such astracking, metering, payment, medical services, remote control, and thelike. More specifically, services provided through MTC may includereading a meter, measuring a water level, utilizing a surveillancecamera, reporting the inventory of a vending machine, and the like. Datacommunication-oriented low-cost/low-specification UEs that provide theseservices are referred to as an MTC UE or low complexity-type UE forconvenience. A BS may determine whether a UE is an MTC UE based on thecapability of the UE. In the present specification, an MTC UE, alow-complexity UE, a low-cost UE, and a UE Category 0 UE may be usedwith the same meaning, and a normal UE may be used to refer to a UEother than the listed UEs.

The MTC server 1330 is an entity communicating with the MTC UE 1310. TheMTC server 1330 runs an MTC application and provides an MTC-specificservice to an MTC device. The MTC UE 1310 is a wireless device thatprovides MTC communication and may be fixed or mobile.

Since an MTC UE has a small amount of data to transmit and isoccasionally involved in uplink/downlink data transmission/reception, itis effective to reduce the cost of the UE and to decrease batteryconsumption thereof according to a low data transmission rate. The MTCUE is characterized by low mobility and thus has a hardly changingchannel environment.

The MTC UE does not require a high-performance function and generallyuses a small amount of data. The concept of UE Category 0 is introducedin order to manufacture a low-cost MTC UE. A UE category is a generalnumber used in the 3GPP to indicate how much data a UE can process in acommunication modem. Table 4 shows 3GPP UE categories.

TABLE 4 UE UE Cate- DL UL Cate- DL UL gory speed speed gory speed speed0  1 Mbps  1 Mbps 7 300 Mbps 100 Mbps 1  10 Mbps  5 Mbps 8 3 Gbps 1.5Gbps 2  50 Mbps 25 Mbps 9 450 Mbps 50 Mbps 3 100 Mbps 50 Mbps 10 450Mbps 100 Mbps 4 150 Mbps 50 Mbps 11 600 Mbps 50 Mbps 5 300 Mbps 75 Mbps12 600 Mbps 100 Mbps 6 300 Mbps 50 Mbps 13 400 Mbps 50 Mbps

A UE Category 0 UE is allowed to process only 1 Mbps, making it possibleto manufacture a modem without much effort and high costs, and may useonly one antenna. Also, the UE Category 0 UE is allowed to performtransmission or reception only in a specified time, rather thansimultaneously performing transmission and reception, and thus mayoperate in FDD in the same manner as in TDD. In addition, unlike inexisting TDD, a sufficient switching time of about 1 ms may be assignedfor a period of transition between transmission and reception, therebyremarkably reducing costs for hardware components, particularly in viewof a modem and RF, overall.

MTC UEs may be installed not only in buildings and factories but also incoverage-limited places, for example, a basement. For instance, about20% of MTC UEs supporting an MTC service, such as smart metering, may beinstalled in a poor ‘deep indoor’ environment, such as a basement. Thus,for successful MTC data transmission, it is necessary to increase thecoverage of an MTC UE by about 20 dB as compared with the coverage of aconventional normal UE. Considering this situation, various coverageenhancement techniques are currently under discussion, such as arepetitive transmission method for an MTC UE by each channel/signal.

FIG. 14 shows an example of cell coverage enhancement for an MTC device.

As described above, recently, there is ongoing discussion about variouscoverage enhancement schemes such as a repetitive transmission methodfor an MTC UE for each channel/signal. The coverage enhancement schememay also be applied not only to the MTC UE but also to a normal UE. Thatis, a UE 1410 located in a coverage enhancement region of FIG. 14 may bea normal UE or an MTC UE. According to a UE location in a cell andsignal quality of the UE in the cell, a coverage enhancement level(hereinafter, CE level) may differ. A difference of the CE level impliesthat there is a difference in the number of repetitions (resources,subframes) required for uplink transmission and downlink reception. Froma UE perspective, it is more preferable to reside in a cell requiring asmall number of repetitions for successful uplink transmission anddownlink reception in terms of power consumption. The small number ofrepetitions for the successful uplink transmission and downlinkreception may be further required particularly for the MTC UE. From anetwork perspective, likewise, it is more preferable to serve for the UErequiring the small number of repetitions. However, as described abovein the cell reselection procedure, the number of repetitions requiredfor the successful uplink transmission and downlink reception is notconsidered in the cell reselection procedure. Consequently, it mayresult in reselecting of a cell requiring a great number of repetitions,which may lead to a great amount of battery consumption. Although it ismentioned for clarity of the explanation that the small number ofrepetitions for the successful uplink transmission and downlinkreception is more required particularly for the MTC device, thetechnical aspect of the present invention is not limited to the MTC UE,and may also be applied to a normal UE located in a coverage enhancementregion.

To solve the aforementioned problem, the present invention proposes acell reselection method considering a CE level or a ranking. It isassumed in the present invention that a plurality of CE levels arepresent for measurement including a level corresponding to a case of nothaving coverage enhancement. According to the CE level, it is assumedthat there is a difference in the number of repetitions required for thesuccessful uplink transmission and downlink reception. The number ofrepetitions may be an amount of resources required for the successfuluplink transmission and downlink reception, and may be the number ofsubframes required for successful uplink transmission and downlinkreception. A CE level 0 may correspond to a case of not having coverageenhancement, and the number of repetitions, the amount of resources, orthe number of subframes required for the successful uplink transmissionand downlink reception may be increased in proportion to an increase inthe CE level.

First, a method of determining whether a UE is a normal mode or a CEmode is described in the present invention.

According to whether the UE is in the normal mode or the CE mode, thecell selection criterion described in Equation 4 may be applieddifferently. The UE of the normal mode implies a UE in normal coverage,and the UE in the CE mode implies a UE in enhanced coverage.

If Srxlev of Equation 2 and Squal of Equation 3 calculated using a firstthreshold satisfy a cell selection condition of Equation 4, it may bedetermined that the UE is in a state of the normal mode. The firstthreshold may be Qrxlevmin or Qqualmin. That is, if cell quality isgreater than or equal to a predefined level and thus successfultransmission/reception is possible even if the number of repetitions fortransmitting/receiving data/signals or the like with respect to a BS isnot greater than or equal to a predefined level, it may be determinedthat the UE is in the state of the normal mode.

Meanwhile, if Srxlev of Equation 2 and Squal of Equation 3 calculatedusing the first threshold do not satisfy the cell selection condition ofEquation 4 but satisfy Srxlev of Equation 2 and Squal of Equation 3calculated using a second threshold satisfy the cell selection conditionof Equation 4, it may be determined that the UE is in the state of theCE mode. The second threshold may be Qrxlevmin_CE or Qqualmin_CE.Qrxlevmin_CE and Qqualmin_CE may be defined as shown in Table 5 below.That is, if the cell quality does not satisfy the predeterminedcriterion and thus successful transmission/reception is possible even ifthe number of repetitions for transmitting/receiving data/signals or thelike with respect to the BS is greater than or equal to a predefinedlevel, it may be determined that the UE is in the state of the CE mode.In case of the UE in the enhanced coverage, Qrxlevmin_CE and Qqualmin_CEmay be used instead of Qrxlevmin and Qqualmin predefined to calculateSrxlev of Equation 2 and Squal of Equation 3. That is, Qrxlevmin_CE andQqualmin_CE may be defined as shown in Table 5 below.

Qrxlevmin UE applies coverage-specific value Qrxlevmin_CE (dBm) QqualminUE applies coverage-specific value Qqualmin_CE (dB)

The UE supporting the CE mode may transition between the CE mode and thenormal mode according to a channel situation with respect to a cell.

Next, a method of determining a CE level is described in the presentinvention. In the present invention, a UE may determine a CE level fortransmission/reception in a specific cell by using a method describedbelow, and each threshold may be provided by a serving cell.

(1) RSRP/RSRQ-based CE level determination: The UE may determine the CElevel of the cell by comparing a measured RSRP/RSRQ result and apredetermined threshold.

In order for the UE to determine a CE level in a specific cell, anetwork may configure an RSRP/RSRQ threshold for one or more CE levels.For example, the network may signal an RSRP/RSRQ threshold 0 for a CElevel 0, an RSRP/RSRQ threshold 1 for a CE level 1, an RSRP/RSRQthreshold 2 for a CE level 2, and an RSRP/RSRQ threshold 3 for a CElevel 3. The level 0 implies that there is no coverage enhancement formeasurement.

While performing measurement on a serving cell and a neighboring cell,the UE may determine a CE level by comparing the threshold configured bythe network and an RSRP/RSRQ result measured by the UE. If themeasurement result is higher than the RSRP/RSRQ threshold 0, the UE maydetermine the CE level to 0. If the measurement result is lower than theRSRP/RSRQ threshold 0 and higher than the RSRP/RSRQ threshold 1, the UEmay determine the CE level to 1. If the measurement result is lower thanthe RSRP/RSRQ threshold 1 and higher than the RSRP/RSRQ threshold 2, theUE may determine the CE level to 2. Likewise, if the measurement resultis lower than the RSRP/RSRQ threshold 2 and higher than the RSRP/RSRQthreshold 3, the UE may determine the CE level to 3.

(2) Primary synchronization signal (PSS)/secondary synchronizationsignal (SSS)-based CE level determination: The UE may determine the CElevel of the cell by comparing a time for acquiring PSS/SSS and apredetermined threshold.

In order for the UE to determine a CE level in a specific cell, anetwork may configure an RSRP/RSRQ threshold for one or more CE levels.For example, the network may signal a time threshold 0 for a CE level 0,a time threshold 1 for a CE level 1, a time threshold 2 for a CE level2, and a time threshold 3 for a CE level 3. The level 0 implies thatthere is no coverage enhancement for measurement.

While performing measurement on a serving cell and a neighboring cell,the UE may determine a CE level by comparing a time threshold configuredby the network and a time for acquiring the PSS/SSS. If the time foracquiring the PSS/SSS is shorter than a time threshold 0, the UE maydetermine the CE level to 0. If the time for acquiring the PSS/SSS islonger than a time threshold 0 and shorter than a time threshold 1, theUE may determine the CE level to 1. If the time for acquiring thePSS/SSS is longer than a time threshold 1 and shorter than a timethreshold 2, the UE may determine the CE level to 2. If the time foracquiring the PSS/SSS is longer than a time threshold 2 and shorter thana time threshold 3, the UE may determine the CE level to 3.

(3) Downlink message-based CE level determination: The UE may determinethe CE level of the cell by comparing the number of repetitions requiredto successfully receive a certain downlink message and a predeterminedthreshold.

(4) Uplink message-based CE level determination: The UE may determinethe CE level of the cell by comparing the number of repetitions requiredto successfully receive a certain uplink message and a predeterminedthreshold.

Although it is assumed in the embodiment of the present invention thatthe CE level can be configured from 0 to 3, this means that one or morelevels can be configured, and the present invention is not limitedthereto.

Next, a cell reselection method proposed in the present invention isdescribed. Proposed are a first method considering not only a prioritybut also a CE level in cell reselection, a second method consideringserving cell quality or the like in cell reselection, and a third methodconsidering only a ranking without consideration of the priority in cellreselection. Hereinafter, each method is described in detail.

1. The first method may consider not only a frequency priority but alsoa CE level in a cell reselection procedure.

(1) Step 1: The UE may perform the cell reselection procedure byconsidering a priority according to the conventional cell reselectioncriterion evaluation. According to the conventional cell reselectioncriterion evaluation, the CE level is not considered in the cellreselection criterion evaluation. The conventional cell reselectionevaluation has already been described above by using FIG. 12.

(2) Step 2: For one or more cells satisfying the criterion of Step 1,the UE may perform cell reselection with an RAT/frequency required tohave a minimum number of repetitions. That is, the UE may perform cellreselection on a cell having a lowest CE level among the cellssatisfying the criterion of Step 1. Alternatively, the UE may performcell reselection on a cell having a highest ranking among the cellssatisfying the criterion of Step 1.

Referring to FIG. 15, there are one serving cell and two neighboringcells. It is assumed that a first neighboring cell has a higher prioritythan a serving cell, whereas a second neighboring cell has a lowerpriority than the serving cell. In addition, it is assumed that a CElevel is 3 as a result of measuring the serving cell, is 2 as a resultof measuring the first neighboring cell, and is 1 as a result ofmeasuring the second neighboring cell. In addition, it is assumed thatthe serving cell, the first neighboring cell, and the second neighboringcell satisfy the condition of Step 1. In this embodiment, although cellreselection is performed on the first neighboring cell having a toppriority according to the conventional cell reselection procedure, cellreselection is performed on the second neighboring cell having a lowestCE level in the present invention. That is, a cell of a frequency havinga low priority may be selected.

(3) Step 3: If there is a plurality of different cells satisfying thecriterion of Step 2, cell reselection with an RAT/frequency having ahigh priority may be preferential over cell reselection with anRAT/frequency having a low priority.

Referring to FIG. 16, there are one serving cell and two neighboringcells. It is assumed that a first neighboring cell has a higher prioritythan a serving cell, whereas a second neighboring cell has a lowerpriority than the serving cell. In addition, it is assumed that a CElevel is 3 as a result of measuring the serving cell, and is 2 equallyas results of measuring the first neighboring cell and the secondneighboring cell. In this case, since the first neighboring cell and thesecond neighboring cell have the same CE level, cell reselection isperformed on the first neighboring cell having a higher priority thanthe second neighboring cell. That is, a cell of a frequency having ahigh priority may be selected in case of the same CE level.

2. The second method may consider not only a priority but also qualityof a serving cell in the cell reselection procedure.

(1) Step 1: If an E-UTRAN frequency or inter-RAT frequency has a higherpriority than a serving frequency, if one second elapses after a UEmoves to a current serving cell, and if any one of the following threeconditions is satisfied during a timer duration Treselection, then cellreselection may be performed on a cell at an E-UTRAN frequency orinter-RAT frequency having a higher priority.

-   -   Condition 1: reception quality (i.e., Squal) of a neighboring        cell−reception quality of a serving cell>a threshold        Thresh.HighQ    -   Condition 2: reception power of a neighboring cell (i.e.,        Srxlev)−reception power of a serving cell>a threshold        Thresh.HighP    -   Condition 3: a CE level of a serving cell−a CE level of a        neighboring cell>a threshold Thresh.HighLevel

That is, at least any one of the reception quality of the serving cell,the reception power of the serving cell, and the CE level of the servingcell may be used in the cell reselection procedure. If the receptionquality of the neighboring cell is greater by at least a specific valueas a result of comparing the reception quality of the neighboring celland the reception quality of the serving cell, the neighboring cell is acell satisfying the condition of Step 1. Alternatively, if the receptionpower of the neighboring cell is greater by at least a specific value asa result of comparing the reception power of the neighboring cell andthe reception power of the serving cell, the neighboring cell is a cellsatisfying the condition of Step 1. Alternatively, if the CE level ofthe neighboring cell is greater by at least a specific value as a resultof comparing the CE level of the neighboring cell and the CE level ofthe serving cell, the neighboring cell is a cell satisfying thecondition of Step 1. The threshold Thresh.HighQ, the thresholdThresh.HighP, and the threshold Thresh.HighLevel may be predefinedvalues.

Cell reselection may be performed on the cell at the E-UTRAN frequencyhaving the same priority on the basis of a ranking for intra-frequencycell reselection.

If the E-UTRAN frequency or the inter-RAT frequency has a lower prioritythan the serving frequency, if one second elapses after the UE moves toa current serving cell, and if any one of the following three conditionsis satisfied during a Treselection timer duration, then cell reselectionmay be performed on a cell at an E-UTRAN frequency or inter-RATfrequency having a lower priority.

-   -   Condition 1: reception quality (i.e., Squal) of a neighboring        cell−reception quality of a serving cell>a threshold Thresh.LowQ    -   Condition 2: reception power of a neighboring cell (i.e.,        Srxlev)−reception power of a serving cell>a threshold        Thresh.LowP    -   Condition 3: a CE level of a serving cell−a CE level of a        neighboring cell>a threshold Thresh.LowLevel

That is, at least any one of the reception quality of the serving cell,the reception power of the serving cell, and the CE level of the servingcell may be used in the cell reselection procedure. If the receptionquality of the neighboring cell is greater by at least a specific valueas a result of comparing the reception quality of the neighboring celland the reception quality of the serving cell, the neighboring cell is acell satisfying the condition of Step 1. Alternatively, if the receptionpower of the neighboring cell is greater by at least a specific value asa result of comparing the reception power of the neighboring cell andthe reception power of the serving cell, the neighboring cell is a cellsatisfying the condition of Step 1. Alternatively, if the CE level ofthe neighboring cell is greater by at least a specific value as a resultof comparing the CE level of the neighboring cell and the CE level ofthe serving cell, the neighboring cell is a cell satisfying thecondition of Step 1. The threshold Thresh.LowQ, the thresholdThresh.LowP, and the threshold Thresh.LowLevel may be predefined values.

(2) Step 2: For one or more cells satisfying the criterion of Step 1,the UE may perform cell reselection with an RAT/frequency required tohave a minimum number of repetitions or a lowest CE level.

(3) Step 3: If there is a plurality of different cells satisfying thecriterion of Step 2, cell reselection with an RAT/frequency having ahigh priority may be preferential over cell reselection with anRAT/frequency having a low priority.

3. The third method may consider only a ranking without having toconsider a priority in the cell reselection procedure.

Cell reselection may be performed on a cell having a highest rankingamong measured cells (frequencies) on a ranking basis. It may beregarded that all measured cells (frequencies) have the same priority.In other words, in the method 3, the priority is used for cellmeasurement, but is not used for cell reselection. A cell having ahighest ranking may be a cell having a lowest CE level.

Similarly to the method 1 and the method 2, in the method 3, cellreselection may be performed on a cell having a highest ranking during atimer duration Treselection and when one second elapses after the UEmoves to a current serving cell.

The method 3 may not be applied to a UE in a normal mode, and maysupport only a UE supporting only a CE mode and in the CE mode. That is,a UE which does not support the CE mode, or which supports the CE modebut is in a state of the normal mode may perform cell reselection byusing the conventional cell reselection procedure. In the UE operatingin the CE mode, a value calculated as a first threshold may not satisfya cell selection condition, and a value calculated as a second thresholdmay satisfy the cell selection condition. The cell selection conditionis satisfied when values Srxlev and Squal exceed 0. The first thresholdis Qrxlevmin and Qqualmin for calculating the values Srxlev and Squal innormal coverage. The second threshold is Qrxlevmin_CE and Qqualmin_CEfor calculating the values Srxlev and Squal in enhanced coverage. Thefirst threshold may be greater than the second threshold. That is, thefirst threshold and the second threshold may be applied to Equation 2and Equation 3 above. The UE in the CE mode may calculate Srxlev andSqual by using any one of Qrxlevmin/Qqualmin or Qrxlevmin_CE/Qqualmin_CEnewly defined for CE.

A threshold used for the cell measurement/ranking may be newly defined.The threshold (e.g., Srxlev or Squal) may be different from that used ina normal cell reselection procedure (including cell measurement).Therefore, a normal mode UE residing in a current cell may select a setof certain parameters, whereas a CE mode UE residing in the current cellmay select a set of other parameters.

FIG. 17 is a block diagram of a method of reselecting a cell by a UEaccording to an embodiment of the present invention. The UE may notconsider a priority in cell reselection.

Referring to FIG. 17, the UE may perform cell measurement on the basisof the priority (S1710). The UE may be a UE operating in a CE mode. Thecell measurement may be performed in the CE mode for a longer time thanin a normal mode.

The UE may calculate a ranking of a cell by using a measurement resultof the measured cell (S1720).

The UE may perform the cell reselection on a cell having a highestranking (S1730). The cell having the highest ranking may be a cellhaving a highest ranking among neighboring cells having a higher rankingthan a serving cell. The cell reselection may be performed for a timeduration Treselection when one second elapses after the UE moves to theserving cell.

FIG. 18 is a block diagram of a method of reselecting a cell by a UEaccording to an embodiment of the present invention. The UE may performcell reselection by considering not only a priority but also a CE levelor ranking of a cell.

Referring to FIG. 18, the UE may perform cell measurement and cellreselection criterion evaluation on the basis of the priority (S1810).

The UE may be an MTC UE operating in a CE more, and the cell measurementmay be performed in the CE mode for a longer time than in a normal mode.

In case of a neighboring cell having a higher priority than the servingcell, if one second elapses after the UE moves to the serving cell andif a value obtained by subtracting reception quality of the serving cellfrom reception quality of the neighboring cell is greater than athreshold Thresh,HighQ during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. Alternatively, in case of the neighboring cell having thehigher priority than the serving cell, if one second elapses after theUE moves to the serving cell and if a value obtained by subtracting thereception power of the serving cell from the reception power of theneighboring cell is greater than a threshold Thresh,HighP during thetimer duration Treselection, then the neighboring cell may be a cellsatisfying the cell reselection criterion evaluation. Alternatively, incase of the neighboring cell having the higher priority than the servingcell, if one second elapses after the UE moves to the serving cell andif a value obtained by subtracting a CE level of the neighboring cellfrom a CE level of the serving cell is greater than a thresholdThresh,HighLevel during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. The timer Treselection, the threshold Thresh,HighQ, thethreshold Thresh,HighP, and threshold Thresh,HighLevel may be receivedfrom the serving cell.

In case of a neighboring cell having a lower priority than the servingcell, if one second elapses after the UE moves to the serving cell andif a value obtained by subtracting reception quality of the serving cellfrom reception quality of the neighboring cell is greater than athreshold Thresh,LowQ during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. Alternatively, in case of the neighboring cell having thelower priority than the serving cell, if one second elapses after the UEmoves to the serving cell and if a value obtained by subtracting thereception power of the serving cell from the reception power of theneighboring cell is greater than a threshold Thresh,LowP during thetimer duration Treselection, then the neighboring cell may be a cellsatisfying the cell reselection criterion evaluation. Alternatively, incase of the neighboring cell having the lower priority than the servingcell, if one second elapses after the UE moves to the serving cell andif a value obtained by subtracting a CE level of the neighboring cellfrom a CE level of the serving cell is greater than a thresholdThresh,LowLevel during the timer duration Treselection, then theneighboring cell may be a cell satisfying the cell reselection criterionevaluation. The timer Treselection, the threshold Thresh,LowQ, thethreshold Thresh,LowP, and threshold Thresh,LowLevel may be receivedfrom the serving cell.

The UE may calculate the CE level by using a cell measurement resultwith respect to the cell satisfying the cell reselection criterionevaluation (S1820).

The CE level may be calculated on the basis of measured reference signalreceived quality (RSRQ) or reference signal received power (RSRP), and acell having the lowest CE level may be a cell having the best measuredRSRQ or RSRP among cells satisfying the cell reselection criterionevaluation. The CE level may be calculated on the basis of a time forobtaining a primary synchronization signal (PSS) or a secondarysynchronization signal (SSS), and a cell having the lowest CE level maybe a cell having a smallest time for obtaining the PSS or the SSS amongthe cells satisfying the cell reselection criterion evaluation. The CElevel may be calculated on the basis of the number of repetitions forreceiving a downlink message, and the cell having the lowest CE levelmay be a cell having the smallest number of repetitions for receivingthe downlink message among the cells satisfying the cell reselectioncriterion evaluation. The CE level may be calculated on the basis of thenumber of repetitions for transmitting an uplink message, and the cellhaving the lowest CE level may be a cell having the smallest number ofrepetitions for transmitting the uplink message among the cellssatisfying the cell reselection criterion evaluation.

The UE may perform cell reselection on a cell having a lowest CE level(S1830).

The cell having the lowest CE level may be a cell having a highestranking. If there is a plurality of cells having a lowest CE level, thecell having the lowest CE level may be a cell having a top priorityamong the cells having the lowest CE level.

FIG. 19 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

A BS 1900 includes a processor 1901, a memory 1902 and a transceiver1903. The memory 1902 is connected to the processor 1901, and storesvarious information for driving the processor 1901. The transceiver 1903is connected to the processor 1901, and transmits and/or receives radiosignals. The processor 1901 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the basestation may be implemented by the processor 1901.

A UE 1910 includes a processor 1911, a memory 1912 and a transceiver1913. The memory 1912 is connected to the processor 1911, and storesvarious information for driving the processor 1911. The transceiver 1913is connected to the processor 1911, and transmits and/or receives radiosignals. The processor 1911 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the UE may beimplemented by the processor 1911.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The transceiver may include abase-band circuit for processing a wireless signal. When the embodimentis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory andmay be performed by the processor. The memory may be located inside oroutside the processor, and may be coupled to the processor by usingvarious well-known means.

Various methods based on the present specification have been describedby referring to drawings and reference numerals given in the drawings onthe basis of the aforementioned examples. Although each method describesmultiple steps or blocks in a specific order for convenience ofexplanation, the invention disclosed in the claims is not limited to theorder of the steps or blocks, and each step or block can be implementedin a different order, or can be performed simultaneously with othersteps or blocks. In addition, those ordinarily skilled in the art canknow that the invention is not limited to each of the steps or blocks,and at least one different step can be added or deleted withoutdeparting from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should benoted that those ordinarily skilled in the art know that all possiblecombinations of examples cannot be explained, and also know that variouscombinations can be derived from the technique of the presentspecification. Therefore, the protection scope of the invention shouldbe determined by combining various examples described in the detailedexplanation, without departing from the scope of the following claims.

What is claimed is:
 1. A method of reselecting a cell, which belongs toan inter-frequency, by a user equipment (UE) in an enhanced coverage,the method comprising: based on a frequency priority, measuring thecell, which belongs to the inter-frequency, to obtain a cell measurementresult, wherein a frequency priority of the inter-frequency to which thecell belongs is not equal to a frequency priority of a serving frequencyto which a serving cell belongs; obtaining a ranking of the cell basedon the cell measurement result and a ranking of the serving cell; andbased on the obtained rankings, reselecting one cell having a bestranking among the cell and the serving cell.
 2. The method of claim 1,wherein when the cell has a higher ranking than the serving cell duringa timer duration and when a specific time duration elapses after the UEmoves to the serving cell, the one cell having a best ranking isreselected among the cell and the serving cell.
 3. The method of claim1, wherein the frequency priority is not considered by the UE when theUE reselects the one cell.
 4. The method of claim 1, wherein the onecell is a cell having a highest ranking among neighboring cells having ahigher ranking than the serving cell.
 5. The method of claim 1, whereinthe obtaining a ranking of the cell comprises: determining whether thecell meets a first threshold that does not satisfy a cell selectioncondition or a second threshold that does satisfy the cell selectioncondition.
 6. The method of claim 5, wherein the cell selectioncondition is satisfied when values Srxlev and Squal exceed 0, whereinthe first threshold is Qrxlevmin and Qqualmin for obtaining the valuesSrxlev and Squal in normal coverage, wherein the second threshold isQrxlevmin_CE and Qqualmin_CE for obtaining the values Srxlev and Squalin enhanced coverage, and wherein the first threshold is greater thanthe second threshold.
 7. The method of claim 1, wherein the measuringthe cell comprises: measuring the cell in the enhanced coverage for alonger time than in a normal mode.
 8. A user equipment (UE) that is inan enhanced coverage for reselecting a cell which belongs to aninter-frequency, the UE comprising: a memory; a transceiver, and aprocessor operatively coupled to the memory and the transceiver, whereinthe processor is configured for: based on a frequency priority,measuring the cell, which belongs to the inter-frequency, to obtain acell measurement result, wherein a frequency priority of theinter-frequency to which the cell belongs is not equal to a frequencypriority of a serving frequency to which a serving cell belongs;obtaining a ranking of the cell based on the cell measurement result anda ranking of the serving cell; and based on the obtained rankings,reselecting one cell having a best ranking among the cell and theserving cell.
 9. The UE of claim 8, wherein when the cell has a higherranking than the serving cell during a timer duration and when aspecific time duration elapses after the UE moves to the serving cell,the one cell having a best ranking is reselected among the cell and theserving cell.
 10. The UE of claim 8, wherein the frequency priority isnot considered by the UE when the UE reselects the one cell.
 11. The UEof claim 8, wherein the one cell is a cell having a highest rankingamong neighboring cells having a higher ranking than the serving cell.12. The UE of claim 8, wherein the obtaining a ranking of the cellcomprises: determining whether the cell meets a first threshold thatdoes not satisfy a cell selection condition or a second threshold thatdoes satisfy the cell selection condition.
 13. The UE of claim 12,wherein the cell selection condition is satisfied when values Srxlev andSqual exceed 0, wherein the first threshold is Qrxlevmin and Qqualminfor obtaining the values Srxlev and Squal in normal coverage, whereinthe second threshold is Qrxlevmin_CE and Qqualmin_CE for obtaining thevalues Srxlev and Squal in enhanced coverage, and wherein the firstthreshold is greater than the second threshold.
 14. The UE of claim 8,wherein the measuring the cell comprises: measuring the cell in theenhanced coverage for a longer time than in a normal mode.
 15. Themethod of claim 1, further comprising: communicating with a base stationvia the reselected cell.
 16. The UE of claim 8, wherein the processor isconfigured for communicating with a base station via the reselectedcell.